Shares of Robinhood Markets Inc. /zigman2/quotes/228268942/composite HOOD -0.85% skyrocketed to price that was more than double its initial public offering price in very volatile early trading Wednesday, before paring gains to be up "just" 40%. The stock rallied 81.6% to an intraday high of $85 within the first 15 minutes after the open, which was 123% above the $38 IPO price, but has since pulled back to be up 38.5% at $64.12. That follows a 24.2% gain on Tuesday, when it closed above its IPO price for the first time, on Robinhood's fourth day as a public company. The stock has already been halted three times for volatility, all in the first half-hour of the regular session. With 55.4 million shares already traded, the stock is the second-most active on major U.S. exchanges. The surge in shares of Robinhood, a popular place to trade so-called "meme stocks," is coming on a day in which the meme stocks AMC Entertainment Holdings Inc. /zigman2/quotes/200235402/composite AMC -0.44% fell 1.8% and GameStop Corp. /zigman2/quotes/203755179/composite GME -4.76% gained 1.7%, while the S&P 500 /zigman2/quotes/210599714/realtime SPX +0.08% slipped 0.2%.